1. Field of the Invention
The present invention relates to an electro-acoustic transducer which is mounted to, for example, a cellular phone or a pager, and which is utilized for reproduction of alarm sounds, melody sounds and speech sounds at the time of reception of a call, and also relates to an electronic device such as a cellular phone, a PDA (personal digital assistant), a TV, a personal computer, a car navigation system and the like, wherein such an electro-acoustic transducer is built in.
2. Description of the Related Art
Reduction in the thickness and reduction in the amount of power consumed of electronic device, as represented by cellular phones, PDAs and the like, has been progressing and further reduction in thickness as well as further enhancement in efficiency in electro-acoustic transducers mounted to such electronic device are likewise desirable. Hence, an electro-acoustic transducer, as shown in FIG. 1, has been invented in order to achieve reduction in thickness and enhancement in efficiency (Japanese Unexamined Patent Publication No. H8(1996)-140185).
In this electro-acoustic transducer a casing 20 is formed of a cylindrical cover 1, of which one end is opened, and of a cylindrical frame 2, of which one end is also opened, that are connected to each other. A plurality of small holes 11 are provided in a circular form in the cover 1 for the release of sound. A magnet 3 is secured in a coaxial manner with the cover 1 inside of the cover 1.
A diaphragm 4 in a disk form is placed inside of the casing 20 so as to have a gap G between the diaphragm 4 and the lower surface of the magnet 3, wherein the outer periphery portion of the diaphragm 4 is placed and secured between the cover 1 and the frame 2. A drive coil 5 is secured on the lower surface of the diaphragm 4 so as to be coaxial with the magnet 3. An electrode 6 that allows a current to flow through the drive coil 5 is secured at the bottom surface of the frame 2. A lead wire (not shown) from the drive coil 5 is connected to an edge portion of the electrode 6.
In such an electro-acoustic transducer magnetic flux is emitted from the magnet 3 in a direction approximately perpendicular to the surface of the magnet from the center portion of the magnet so as to penetrate the drive coil 5. On the other hand, the magnetic flux spreads from the surface of the periphery portion of the magnet in a radial form so as to diagonally penetrate the drive coil 5. When a current flows through the drive coil 5 in such a magnetic field, a driving force generates so as to be applied to the drive coil 5 in the direction perpendicular to the diaphragm 4 so that the diaphragm vibrates upwardly and downwardly resulting in the generation of sound. In the case of this electro-acoustic transducer a yoke or a center pole become unnecessary due to the direct emission of magnetic flux from the magnet, thereby the thickness of the transducer can be reduced. In addition, because the winding width of the drive coil 5 can be freely determined, the impedance value can be controlled and, as a result, the amount of power consumption can reduce due to high impedance.
The driving force generating in the drive coil 5, however, is proportional to the magnetic flux perpendicular to the direction of the current flowing through the drive coil 5 and perpendicular to the direction of vibration of the diaphragm 4. Since the magnetic flux parallel to the direction of vibration, rather than the magnetic flux perpendicular to the direction of vibration, is dominant in the conventional electro-acoustic transducer, a sufficient driving force is low and sound pressure of reproduced sound becomes low.
In addition, the magnetic flux emitted from the magnet decreases in proportion to the distance from the magnet. That is, the driving force generating in the drive coil differs between the case where the diaphragm vibrates from the neutral position to the upward direction, that is, in the direction going away from the magnet, and the case where the diaphragm vibrates from the neutral position to the downward direction, that is, in the direction approaching the magnet. There is a problem that this asymmetry causes distortion to the driving force so that the reproduced sound deteriorates.
In addition, in the case of a general electrodynamic type electro-acoustic transducer, a drive coil is inserted into a magnetic gap in a magnetic circuit formed of a magnet, a yoke and a center pole. Therefore, in the case that a drive coil having an unequal form, such as an elliptical or a rectangular form, in comparison with a circular form is inserted into a magnetic gap, the drive coil easily makes contact with the magnetic gap when vibrating. That may cause, in some cases, abnormal sound. Widening of the magnetic gap in order to avoid this phenomenon leads to a reduction in the sound pressure of the reproduced sound. Therefore, there is a limitation to the aspect ratio when the form of such an electrodynamic type electro-acoustic transducer is made elliptical or rectangular.